Phosphates are common in nature. They exist as the polar hydrophilic head groups in the phospholipid bilayers of cell walls and in micelles. They are also prevalent in nucleic acids (RNA and DNA), and in the cell's “energy molecule” adenosine triphosphate (ATP). Phosphate minerals in rock like calcium hydroxyphosphate and calcium fluorophosphate are also natural sources.
In aquatic environments, phosphorus is known as the “limiting nutrient.” It combines with nitrogen and photosynthetic processes to produce plant and algal growth. Phosphorus in marine life support systems exists in either a particulate or dissolved phase. While particulate sources may be removed during filtration or fractionation, the dissolved form persists.
Phosphorus is dissolved in water primarily as the phosphate ion (PO43−). Natural or background concentrations of phosphate in water, also known as inorganic or orthophosphate, are in the range 0.005-0.05 mg/L. Periodic algal blooms may be observed at phosphate concentrations between 0.08-0.10 mg/L. Long term eutrophication can be avoided when phosphate levels are kept below 0.5-0.05 mg/L. In marine systems, phosphorus is the limiting nutrient when the nitrogen to phosphorus ratio is greater than 16, and this is almost always the case in aquarium life support systems. Generally, to minimize algal growth on surfaces, life support and water quality managers try to keep phosphate levels less than or equal to about 0.05 mg/L.
In aquarium life support systems, phosphates are present in particulate and dissolved forms. Particulate phosphorus may be removed during filtration, fractionation, and ozonation processes. Dissolved phosphate remains and circulates through the system. It is this form that is the limiting nutrient for algae growth. At pH 8, about 85% of dissolved phosphate is present as hydrogen phosphate ion (HPO42−). The rest is dihydrogen phosphate ion (H2PO4−).
Due to the prevalence of phosphates in water and the potential hazards and expense of cleaning up the results of high concentrations of phosphate, there is a need to develop methods and materials to keep the levels of phosphate at low levels.